IL-10 is a multifunctional cytokine that regulates complex immune responses. Its normal function is to protect the host from uncontrolled inflammatory responses. However, IL-10 has also been implicated as an autocrine growth factor in several B-cell malignancies and stimulates B-cell mediated autoimmune disease. The normal and pathological functions of IL-10 are initiated by IL-10 receptor engagement and assembly into a signaling competent IL-10/IL-10R1/IL-10R2 complex. In addition to cellular IL-10 (clL-10), Epstein Barr virus (EBV) and cytomegalovirus (CMV) harbor viral IL-10 mimics (ebvlL-10 and cmvlL-10) in their genomes that activate the IL-10 signaling complex, resulting in overlapping and distinct biological properties. In the past funding period, we determined crystal structures of clL-10, cmvlL-10, and ebvlL-10 bound to the high affinity IL-10R1 chain. In this proposal we will use surface plasmon resonance, site-directed mutagenesis, NMR spectroscopy, X-ray crystallography, and FRET methods to study cellular and viral IL-10 receptor interactions. These studies will be complemented by the analysis of the cellular IL-10 homologs IL- 22 and IL-20. The long term goal of this proposal is to derive a quantitative structural/computational model of IL-10 family signaling that might explain how cellular and viral IL-10s shape immune responses and allow the rational design of cytokine therapeutics.